Monday, April 24, 2017

Vacuum tube curve tracer completed and working perfectly


I installed the barrier strip, insulated joints, tied down the fuse holders and choke inside, trimmed up connections, and put the entire unit together in its final configuration. It sits in the wooden case of a Century Tube Tester that I repurposed to hold my project, leaving some items like the meter and unused sockets because the cosmetics would be worse with all the gaping holes.

Completed curve tracer in its wooden case
The side panel has the connector for an RS232 serial link to the PC, a switch to choose either anode/screen reservoir voltages or the barrier terminal block to connect to special positions 10 and 11 on the rotary switches.

Side connections - serial port, special voltage switch and external heater block
The power brick and the USB to RS232 adapter cords sit in the slot on the left of the faceplate. The nine rotary knobs choose the appropriate connection for each of the 9 possible pins on a tube socket. Only five of the sockets are connected, the rest are dummies.  The black and red top hat connection cables allow grid and anode sources, respectively, to be connected to any tube with a top hat connector. 
Completed and assembled tracer
The Quality light is actually the power on LED showing the circuit board is powered up. The Short light is actually the "high voltage present" LED which indicates that the screen and anode reservoirs have hundreds of volts present. The small 'power' light is not active, nor is the meter on the top.

I fired up the tracer, hooked it to the PC, fired up the GUI control software, and inserted a 12AU7 tube in one of the active 9 pin sockets. I rotated the appropriate switches to connect the target pins to the voltage they represent. 

For example, the second triode in the 12AU7 (it is a dual-triode tube) has the plate connected to pin 1, the grid to pin 2 and the cathode to pin 3. The filament is 12.6V across pins 4 and 5. Thus, switch 1 was rotated to the position for anode voltage (position 8), switch 2 to the position for grid voltage (position 6), switch 4 rotated to filament A (position 2), etc. 

I ran the 'quick test' on the tube, which is set to the voltages from the data sheet where characteristics such as plate resistance, transconductance and amplification factor are reported. In this case, the manufacturer's data sheet showed that with 250 volts on the anode and -8.5 volts on the grid, it should draw 10.5ma on the plate, have a plate resistance of 7.7K ohms, a transconductance of 2200 uMho and an amplification factor of 17. 

I entered the two settings and the nominal values into the quick test form. It showed the plate current a bit high, the plate resistance exactly on spec, and the transconductance and amplification factor about 9% low compared to spec. Reasonable results for a used tube.

I then ran a curve trace to match the conditions for one of the curves illustrated in the data sheet. The curves for 100, 150, 200 and 250 volts on the anode where shown while varying the grid from -18 to 0 volts, displaying the plate current against grid voltage for the four curves. 
The expected performance of a new 12AU7 tube
My curves are similar but not identical, consistent with the degradation of the used tube but similar enough to be a reasonable test of this curve tracer. I could have moved the plate current scale to the right side and adjusted the start/stop values of the axes to line up exactly with the factory drawn curve, if that was important. I can also hide the measurement points and just leave the interpolated curves. 
Measured curve of one of the triodes in my used 12AU7
This project is complete! Next up I started through all the tubes from my Heathkit HW-100 transceiver, to check them against spec. Might as well play with different curve plots and options too.

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